1. Field of the Invention
The present invention relates to an electron microscope that uses an electron beam to observe a specimen, and for example to a scanning electron microscope that detects electrons emitted from a specimen under electron beam irradiation to measure or examine the shape and composition of the specimen.
2. Background Art
Miniaturization and large-scale integration of ULSI (Ultra-Large Scale Integration) elements have been accelerated, so that devices are now processed at dimensions of several tens of nanometers. Furthermore, there have been increasingly developed multilayered devices using various types of new materials, such as low dielectric constant films and metal gate films to achieve higher speeds, as well as three-layer resist films to enhance etching resistance. To this end, tighter critical dimension (CD) management during ULSI processing has been required.
In semiconductor processing steps, insulators, such as resist films, insulating films, and low-k materials, are frequently used. When the surface of such an insulator (specimen) is irradiated with an electron beam, the surface is charged. The charged specimen not only changes the amount of secondary electrons that attempt to escape from the surface of the specimen but also bends the trajectory of the primary electron beam, resulting in distortion of images obtained by a scanning electron microscope. As a result, it is difficult to measure true dimensions and shapes after processing. For example, in the case of an ArF resist, it cannot be judged whether line edge roughness (LER) has occurred in an etching step or charging in the electron microscope has caused an error in dimension measurement. There are also problems in observation of contact hole having a large aspect ratio, such as distortion of the observed shape of the contact hole, and difficulty in identifying whether the observed hole diameter is either the upper or lower diameter. The charged state not only spatially changes due to the motion and diffusion of the electrons but also spatially and temporally changes due to, for example, hole-electron recombination-related attenuation. Furthermore, the specimen may be “positively charged” or “negatively charged” depending on observation conditions (the energy of the electron incident on the surface of the specimen, the magnification, the amount of electric current, and the scanning method). It is therefore important to control the charged state.
Conventional methods for controlling the charged state are summarized as follow: For example, JP Patent Publication (Kokai) No. 5-151927 (1993) (corresponding to U.S. Pat. No. 5,412,209) describes a method for stably acquiring images by performing primary electron irradiation at a low magnification different from that for observation (pre-dose) to intentionally charge an area larger than the observation area so as to obtain a uniformly charged observation area. In JP Patent Publication (Kokai) No. 7-14537 (1995), positive or negative charging is cancelled or reduced to alleviate the influence of the charging on observation by in advance performing an electron beam irradiation having energy different from that of the primary electron beam for observation. In JP Patent Publication (Kohyo) No. 2001-508592 (corresponding to U.S. Pat. No. 6,211,518), which discloses a similar technology, the observation area and a larger surrounding area are flooded (irradiated) with positive electric charge during a plurality of frame cycles to reduce the voltage difference between the scanned area and the surrounding area, and then the observation area is separately scanned to obtain a bright image of the observation area. JP Patent No. 3238705 (corresponding to U.S. Pat. No. 5,302,828) discloses a scanning technology by which the influence of surface electric charge is reduced when a substantially circular hole is scanned. There is another method in which the center position of a hole is identified in a low magnification, and the electron beam scan is repeatedly carried out from the center of the hole to a point beyond the edge of the hole in the outward radial direction in a high magnification. These technologies are designed to reduce the influence of charging and stably acquire accurate images.
JP Patent Publication (Kokai) No. 62-52841 (1987) discloses a method for measuring charging potential including the steps of determining the time constant of brightness from change in brightness of each of a plurality of images and outputting these time constants as potential information.